In oil fields, the reservoir bed belonging to the continental facies bears considerable heterogeneity during the process of sedimentation. Part of the crude oil containing water has fast rising velocity for its high viscosity. In addition, the exploitation of many oil fields has entered into the middle or later period: though a series of measures such as water or steam injection have been taken, the recovery efficiency of crude oil is still low, the exploitation cost is high, and a lot of original oil in-place is difficult to or even cannot be exploited economically. Worse still, the recovery efficiency is even lower while the cost is even higher with regard to low permeable oil, thick crude oil as well as crude oil of condensate gas reservoirs.
Natural gas injection into the oil field can be used for oil recovery. Fairly favorable results can be obtained for the following reason: natural gas possesses some properties of crude oil in oil reservoirs and would not cause any harm to the oil bed; consequently, miscibility can be obtained under relatively low pressures or in the process of flooding. However, the technology is, to a certain extent, restricted by economic factors for the rising price of gas hydrate (including natural gas), which, as a result, motivates the studies on the injection of other gases, especially carbon dioxide and nitrogen.
Oil recovery efficiency can be increased by flooding carbon dioxide and/or nitrogen into the oil fields on the basis of traditional steam injection. Having high solubility in both water and oil, carbon dioxide, when solved into crude oil in huge amount, can expand the volume as well as decrease the viscosity of the crude oil. During the process of miscible flooding, carbon dioxide extracts and gasifies the light components contained in the crude oil, and in this way the interfacial tension is reduced and flooding is carried out. Meanwhile, carbon dioxide functions to improve mobility ratio, extract and gasify light hydrocarbons in crude oil as well as increasing, by molecular diffusion, the permeability of oil reservoirs, etc.
Flue gas of combustors such as boilers etc. consists of carbon dioxide and nitrogen, two key and economical gases for oil recovery by gas injection. With coal, gas or oil as the fuel, combustors including boilers used in oil fields usually produce flue gas containing 9%˜15% carbon dioxide (by volume), 0%˜85% nitrogen (by volume) and slight quantity of oxygen, sulfur dioxide and water.
Reclamation of carbon dioxide from flue gas of coal, gas or oil boilers may protect the environment and realize environment-friendly production. As the dominating component of greenhouse gases, carbon dioxide exerts non-negligible negative influence on the global ecological system, on the economic development as well as on the health and life quality of human beings. A critical point for minimizing the greenhouse effect is to restrict the content of carbon dioxide in the atmosphere. Therefore, reclamation of carbon dioxide has tremendous significance, considered from the viewpoint of tackling the greenhouse effect problem. Therefore, it is of great significance to reclaim carbon dioxide from flue gas of combustor so as to apply it in gas injection oil producing technology, which may not only increase the crude oil output but also reduce pollution to the environment.
Nitrogen has low solubility in water and the similar viscosity as methane for its larger compressibility and formation volume factor than other gases. As an inert gas, it has no toxicity or corrosivity or flammability; mixture of nitrogen with most of fluids always presents in the phase of cystose, which may have certain lifting influence on the fluids. The above properties enables nitrogen to improve the recovery efficiency of oil by various flooding modes such as immiscible flooding, miscible flooding, gravity flooding, combination flooding and water-gas alternating flooding, etc.
Thermal recovery has dominated the exploitation mode of some thick crude oilthick crude oil blocks in some oil fields. Take wet steam generator (23 ton thermal recovery boiler) for example: with natural gas as the fuel, each boiler discharges approximately 480 million Nm3 of flue gas per year, which, if used as the proximal gas resource of carbon dioxide and nitrogen, may drastically lower the oil recovery cost and provide optimal resources for oil recovery by flue gas injection.
Gas injection into the oil well is an important technology for improving the oil recovery efficiency by reducing interfacial tension, lowering the viscosity of crude oil and restoring or maintaining the reservoir pressure when gases and crude oil form into immiscible or miscible phase for different gases and under different reservoir conditions.
The primary components of flue gas of steam-injection boilers, in the process of thermal recovery of thick crude oil by steam injection, are carbon dioxide and nitrogen. As is presented by studies, combination of steam boilers and mate flue gas reclamation equipment may realize the selective injection of steam, carbon dioxide, nitrogen or their mixture, which not only acquires satisfactory results of outcome increase but also is conducive to the environment protection.